Thank you Roger,
First, when we have an older cat like Ginger who is failing to keep weight on despite an excellent appetite, we do have a range of issues to consider. We can divide into 3 categories: (1) conditions that cause weight loss due to decreased nutrition intake (obviously not the case with his great appetite), (2) those due to increased output (again less likely if he isn’t vomiting, having diarrhoea. Though we'd not be able to rule out potential urinary protein loss at the moment), and (3) those that do so siphoning the nutrition to support that disease process.
That all said, the last two categories are where our concerns lie for your lad. And this isn't an uncommon scenario for cats his age. The reason is because we can see older cats start struggle to keep weight on despite a good to great appetite for a range of reasons. Worms can play a role but often this is in the younger cat or the elderly stray that has lived on wildlife as their main food source. Issues that can manifest this way at his age will include conditions like hyperthyroidism, diabetes, liver disease, and kidney troubles. As well, while it isn't nice to think about, we must keep in mind that cancer in cats his age can manifest as weight loss with little other signs (just as it doesn't impinge on the body, only to steal nutrients from him).
Therefore, with these in mind, we do want to get to the bottom of his signs. To do so, it would be worth having him checked by his vet (if he is due for a vaccination soon, you could move it up a wee bit early and have him checked out at that time). The vet will be able to have a feel of him and just make sure there are no sinister lumps and bumps to blame for his weight loss. Depending on their findings, you could have the vet check a blood sample to rule out the above concerns. And depending on the results, the vet will be able to advise you on treatment and support for these conditions.
If you weren't keen to have him checked out at this stage, then I would suggest considering having a urine sample tested. Often we can obtain a ‘donation’ if the kitty is left overnight in a non-carpeted room with an empty litter box. The vet will be able to analyze it and determine if there is anything abnormal. They will be able to appreciate changes to the urine's white blood cell content (a marker of infection), the presence of glucose/ketones (markers of diabetes) and bilirubin (a sign of liver disease). Furthermore, the vet will be able to check the urine's specific gravity, which tells us if the kidneys are concentrating the urine properly (since dilute urine is seen with kidney disease). Overall, this is quite a non-invasive means of ruling out a number of the above differentials for Ginger.
Overall, a great appetite isn't a concern if a cat is putting weight on to match their intake or at least not losing weight. For Ginger to eat the dietary requirement of 2-3 of himself and not do so raises red flags here. Therefore, in his case it would be prudent to have him checked out and at least have a urine sample (or blood sample) checked. Because if you are able to the bottom of the exact cause sooner rather then later, it will give you the best chance to deal with it and help support him in a way to either address his elevated nutritional requirements to avoid weight loss or stop the potential disease process triggering this.
I hope this information is helpful.
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